The present invention relates generally to aircraft voice communication radios such as are normally employed in two-way communication with air traffic control facilities. More particularly, the invention relates to means for preventing interference of the type which arises when two or more radios are transmitting on the same channel frequency.
Two-way voice communications between aircraft and ground-based air traffic control facilities are generally carried out by means of VHF communications transceivers, customarily termed "COM" radios. Typically, COM radios employ amplitude modulation (AM) and operate in a frequency band from 118.0 MHz to 135.95 MHz on any of a plurality of selectable discrete channel frequencies, e.g., on any one of 720 defined channels, with 0.025 MHz channel spacing.
At each radio location, whether within an aircraft or at an air traffic control facility, there is typically a manually-operable push-to-talk (PTT) switch connected through a suitable microphone switch circuit for activating the transmitter circuitry and de-activating the receiver circuitry. Thus, the normal or stand-by mode is the receiver mode. In the case of a hand-held microphone, the PTT switch is normally included within a microphone housing including both a microphone element and the PTT switch. A microphone cord includes both audio and PTT control conductors, and terminates in a suitable microphone plug. In other cases, a so-called boom microphone element is attached to a headset assembly, and the microphone switch is located elsewhere, for example, on the aircraft control yoke. In some cases, the microphone switch is even voice activated. In any event, the transmitter circuitry can be activated any time at the discretion of the radio operator, such as the aircraft pilot.
With such a channelized two-way voice communication system, it will be appreciated that only one station at a time can be transmitting on any given channel frequency (within the distance range of the equipment). The consequence of two transmissions at once in many cases is that neither transmission is intelligible at the receiving station. In other words, the particular radio frequency channel is temporarily rendered useless. This is particularly so since amplitude modulation (AM) is employed in the type of aircraft communications radio here concerned, rather than frequency modulation (FM). FM communications systems exhibit a "capture effect" whereby the strongest signal presented to a receiver is heard clearly, to the complete exclusion of the other, weaker transmissions. However, in AM communication systems, all transmissions on a particular channel frequency are heard in the receiver causing mutual interference. Moreover, since the actual carrier frequencies of several transmitters nominally on the same channel frequency are rarely in fact identical, but rather can differ by up to several KHz, Hertz or beat notes in the form of one or more squeals are normally heard in an AM receiver when more than one transmitter is on the same channel frequency, increasing the effective mutual interference.
There are, in general, two types of situations which lead to the undesirable result of two simultaneous transmissions on a single radio frequency channel. One of these types of situations results from poor radio operating technique, and the other results from equipment malfunction.
More particularly, in order to ensure that a particular channel frequency is not already in use, proper radio operating technique is to always listen before transmitting on the particular channel frequency. However, in practice, proper technique is not always followed. Occasionally pilots transmit on a channel frequency at the same time someone else, either an aircraft or a ground facility, is transmitting on the same channel. This is known as "stepping on" or "blocking" the other transmission.
Even if proper radio technique is being employed, equipment malfunction can cause an inadvertent transmission to occur. More specifically, the microphone switch circuit is susceptible to undesired activation. This undesired activation typically occurs as the result of a short circuit in the microphone cord, the connectors, or the push-to-talk switch itself. Also, there are instances where a microphone is not properly returned to its holder, and instead, is left in a location (e.g., on a seat) where the push-to-talk switch is inadvertently actuated through contact with another object. Although one of these typical situations is an equipment malfunction and the other is actually a form of improper operation, the term "Undesired activation" of the push-to-talk switch circuit is herein employed to refer to these and similar situations. Also, it may be noted that the term "stuck microphone" or "stuck mic" is commonly employed to refer to this condition, and the term is also employed herein.
In many two-way communications systems, the channel blockage which results from two simultaneous transmissions on the same channel may be not more than a minor annoyance. However, in the context of an aircraft communication system, such situations are potentially hazardous. This is particularly so because, in busy air traffic control situations, a controller may be communicating in rapid sequence with a number of aircraft employing brief messages and terse phraseology to maximize the amount of information which can be communicated in a short period of time. At aircraft speeds, situations develop rapidly, and it is important that aircraft communications proceed in a smooth fashion. The potentially hazardous consequence of a missed communication during a critical phase of flight will be well appreciated.
As a partial solution to the problems, our previous U.S. patent application Ser. No. 466,318, filed Feb. 14, 1983, entitled "STEP-OFF DEVICE FOR AIRCRAFT VOICE COMMUNICATION SYSTEM", now U.S. Pat. No. 4,494,244, is directed to systems and devices for preventing the operator of an aircraft radio from beginning a transmission while someone else is transmitting on the same channel. Since this improper technique is commonly referred to as "stepping on" a transmission, we have termed the device which prevents this a "step-off" device (SOD).
Briefly, our previously-disclosed step-off device (SOD) is appropriately connected to the receiving circuitry of the aircraft communication radio to sense when a selected radio frequency channel is in use. When the selected channel is in use, operation of the associated transmitter circuitry is inhibited even though the push-to-talk switch is operated. A particularly advantageous form of step-off device is an external device intended for retrofit connection to an aircraft voice communication type which has ground connection-activated push-to-talk switch circuitry, and the external form of step-off device advantageously derives its operating power from the push-to-talk switch circuit. Thus, no modification is required to either the radio receiver wiring or the aircraft electrical system which otherwise might be required for the purpose of deriving operating power, nor are batteries required which would necessitate frequent replacement to maintain proper operation. Additional details may be had by reference to our U.S. Pat. No. 4,494,244, the entire disclosure of which is hereby expressly incorporated by reference.
The step-off device disclosed by our above patent is generally useful, but it does not address the problem of equipment malfunctions which lead to undesired activation of the microphone switch circuit. In the case of a "stuck microphone", that previous step-off device does not prevent a stuck microphone from blocking a particular channel.
As an improvement and modification upon our earlier patent, our U.S. Pat. No. 4,451,854, issued Nov. 5, 1985, and hereby are incorporated by reference, shows an arrangement whereby a "stick microphone reliever" (SMR) is combined with the SOD into a single device referred to as an "anti-blocking device" (ABD). The preferred embodiment of that design uses two semiconductor switches in series with the standard push-to-talk (PTT) switch. One of the semiconductor switches is controlled by the SOD and the other of the semiconductor switches is controlled by the SMR. If the SOD switch opens because the channel is in use, or the SMR switch opens because a condition indicative of a stuck microphone is detected, the transmitter is not enabled upon pushing the PTT switch. The SMR part of that previous design has a timing circuit which opens the SMR semiconductor switch upon transmission continuing for beyond a predetermined interval such as 15 seconds.
Although the ABD of our above patent is a quite useful design, it has been subject to some limitations.
One problem with our previous ABD design is that it may under some conditions penalize the user of the design. For example, assume that two airplanes are trying to communicate upon the same channel and only one of the planes has the previous ABD. The airplane having the ABD will not be able to "step-on" the airplane which does not have the ABD. However, the airplane which does not have the ABD will still be able to "step-on" the transmission from the airplane which has the ABD. By putting someone at a competitive disadvantage for using the safety features of an ABD, one is building in a disincentive for its use. Although government regulation might overcome such a disincentive, it remains a limitation with the previous ABD design.
Our previous ABD design also incorporated an additional manually operable bypass switch to by-pass the ABD if it is malfunctioning or the pilot has an absolute emergency where he must transmit regardless of other signals on the channel.
A further problem of the previous ABD design is that the SMR took a relatively long time interval in order to reset the timer. That is, when the SMR switch opened due to an extended transmission it took a relatively long time for the SMR to recycle such that transmission could start again.
The preferred embodiment of our previous ABD arrangement used two switches in series in order to provide the SOD and SMR functions. The use of two such semiconductor switches increases the complexity of the ABD switching arrangement of the '854 patent beyond that of the simpler SOD only arrangement of our prior '244 patent.
Our previous ABD design further does not provide an audio signal to indicate to a pilot that transmission was inhibited.